fix: correct expToWebhookConfig cache locking

[stoksc]

Ticket

CM-527

Description

There is a write under a read lock, causing the master to crash in integration tests with fatal error: concurrent map writes.

Test Plan

Change is a test fix.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.58%. Comparing base (25ca6d0) to head (f5a7e38).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10014      +/-   ##
==========================================
- Coverage   54.59%   54.58%   -0.02%     
==========================================
  Files        1258     1258              
  Lines      157136   157136              
  Branches     3617     3616       -1     
==========================================
- Hits        85795    85771      -24     
- Misses      71208    71232      +24     
  Partials      133      133              

Flag	Coverage Δ
backend	45.33% <100.00%> (-0.05%)	⬇️
harness	72.74% <ø> (ø)
web	54.34% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
master/internal/webhooks/postgres_webhook.go	45.83% <100.00%> (-0.18%)	⬇️

... and 6 files with indirect coverage changes

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[amandavialva01]

LGTM, good catch!

[gt2345]

Thanks for the fix